A transceiver includes an antenna that is used to transmit and receive radio frequency (RF) signals. The transceiver includes a receive circuit that is connected to the antenna when receiving the RF signals. The receive circuit typically amplifies the received signal from the antenna for further processing. In some applications, it is advantageous to have a receive circuit with an amplifier that has a low input impedance.
For example, current technologies for nuclear magnetic resonance (NMR) transceivers, sometimes referred to as NMR sensors, use the same antenna to generate high-energy RF excitation pulses and detect the echo signals induced by the high-energy RF excitation pulses. NMR transceivers, therefore, face the problem of decoupling a receive circuit driving a amplifier that receives the echo signal provided by the antenna from undesirable ringing effects produced by transient high voltage in the antenna after transmitting excitation pulses. To help dampen residual energy from the antenna after transmitting excitation pulses, an NMR transceiver may have a low input impedance pre-amplifier. Thus, a receiving amplifier with a low input impedance can assist in providing an ultra fast inter echo time (Te) between transmitted excitation pulses and received echo signals.
Though low input impedance receivers can prove advantageous, they also provide challenges. For example, the impedance of a low input impedance receiver loads a source antenna and flattens the frequency response of the source. This makes it difficult to measure the resonance frequency of the antenna. Determining the resonance frequency of the antenna is important since the source antenna resonance frequency is often the frequency at which the best noise performance of a transceiver or transceiver system is achieved. In conventional NMR transceivers, this resonance frequency is typically measured in a receive mode of the transceiver and energy is then transmitted at this frequency in the transmit mode.